As shown in FIG. 1, an existing screw 1 includes a screw head 11 and a threaded shaft 12 extending downwardly from the screw head 11. The threaded shaft 12 has a fixing portion 122 proximate to the screw head 11, and a tapping portion 121 extending from the fixing portion 122 oppositely to the screw head 11. In order to endure stresses induced upon tapping, the existing screw 1 is entirely made from an expensive high strength steel material. However, because only the tapping portion 122, which carries out the tapping operation, requires the high strength material and such a high strength material is not essential for the remaining part of the existing screw, making the existing screw 1 entirely from the high strength material not only results in unnecessary material costs, but also wastes the high strength steel material.